1. Field of the Invention
The present invention relates to an optical glass having optical properties of a low dispersion and a high refractive index.
2. Related Art
In recent years, with a rapid advancement in integration and sophistication of devices using optical systems, there has been an increasing need for high-precision, light-weight, compact optical systems. Among others, due to the popularization of digital cameras, the demand for compact lenses is rising; to minimize the number of lenses, optical designs involving a high refractive index lens becoming the mainstream. In particular, demands are mounting for glass having a higher refractive index and a lower dispersion than conventional glass.
In the case of ordinary high refractive index and low dispersion glass, methods for increasing the refractive index commonly involve the use of rare earth oxides as a component for imparting a high refractive index. In particular, La2O3 is used most often since it facilitates stable production of high refractive index and low dispersion glass.
In recent years, however, needs have been increasing for optical glasses that have a lower dispersion than common optical glass with La2O3 as the main component. To manufacture such a glass, a method was attempted in which part of the La2O3 was replaced with Gd2O3 or the like, taking notice that Gd2O3 or the like was more likely to improve low dispersion properties than La2O3. However, this method failed to prove practical because, in the first place, a simple replacement of a part of or the whole of La2O3 with Gd2O3 or the like complicated the vitrification process tremendously. In addition, this method tended to yield a compromised refractive index; it was difficult to practically achieve low dispersion properties while maintaining a high refractive index. For example, the Patent Document 1 discloses a high refractive index and low dispersion optical glass of a La2O3 system into which Gd2O3 and Y2O3 were introduced. The glass described in this patent, despite achieving a high refractive index of 1.875 or above, provides an Abbe number of around 40, which falls short of sufficiently fulfilling the need of the market. Patent Document 1 Unexamined Japanese Patent Application, First Publication No. 2001-348244